Who was Klaus Fuchs?

A refugee on Britain’s bomb project

Klaus Fuchs was born in 1911 in Rüsselsheim, Germany, the son of a Lutheran theologian. A Communist since his student days, he escaped the Nazis in 1933 and found refuge in Britain. He earned a doctorate in physics at Bristol and worked with Max Born in Edinburgh. In 1940 he was interned in Canada as an enemy alien; released, he joined Rudolf Peierls in Birmingham on Britain’s atomic bomb research in 1941, and became a British subject in 1942. By then he was already passing that research to Soviet intelligence.

Los Alamos and the plutonium design

In late 1943 Fuchs crossed the Atlantic with the British scientists sent to the Manhattan Project, first to New York and, from August 1944, to Los Alamos, New Mexico. He handed papers to a courier he knew only by the cover name Raymond. In June 1945, weeks before the first test in the New Mexico desert, he gave Raymond the design of the plutonium bomb.

On August 29, 1949, the Soviet Union tested its first atomic bomb in Kazakhstan, a close copy of the one dropped on Nagasaki. The CIA had expected it around mid-1953. Historians estimate that Fuchs brought the Soviet bomb forward by one to two years. Moscow tells it with its own emphasis: Stalin made his security chief, Lavrentiy Beria, head of a crash program, and Soviet physicists built the bomb. In November 1947 the foreign minister, Vyacheslav Molotov, told the world the bomb’s secret “has long ceased to exist.”

How the codebreakers found him

Fuchs was caught by another secret. Since 1943 Army codebreakers at Arlington Hall, Virginia, had worked on Soviet cables, and some became readable because Moscow’s clerks had reused pages of their one-time pads. In 1949 a decrypted cable sent from New York in 1944 turned out to carry a stolen British paper on separating uranium. The FBI and Britain’s MI5 traced its author to Fuchs, by then head of theoretical physics at Harwell, Britain’s atomic research center. In the cables his cover name is REST, and later CHARLES.

No court could see those cables without Moscow learning they were being read. So the MI5 officer William Skardon kept coming back to Harwell to talk, and in January 1950 Fuchs told him everything. His written statement explained how he had begun:

At this time I had complete confidence in Russian policy and I believed that the Western Allies deliberately allowed Russia and Germany to fight each other to the death.

On March 1, 1950, at the Old Bailey, he was sentenced to fourteen years, the maximum, because the Soviet Union had been an ally, not an enemy.

The chain that followed

In May 1950 Fuchs identified Raymond in FBI photographs as Harry Gold, a Philadelphia chemist. Gold named an Army machinist at Los Alamos, David Greenglass, and Greenglass pointed to his sister’s husband, Julius Rosenberg, whose case is weighed in Were the Rosenbergs guilty?. The confession also landed two weeks before Senator McCarthy’s speech at Wheeling.

Fuchs was not the only source inside Los Alamos. The cables name Ted Hall, a nineteen-year-old physicist who handed over the same design. Hall was never charged, because the evidence was too secret to show a jury. In 1950 another Harwell physicist, Bruno Pontecorvo, fled to Moscow.

After prison

Britain stripped Fuchs of his citizenship. Released in 1959 after nine years, he left for East Germany, where he became a senior nuclear scientist. He died in East Berlin in 1988. The cables that had found him stayed secret until 1995, when the Venona releases showed how much the codebreakers had known.